Fluid bed granulation process and apparatus

ABSTRACT

Fluid bed granulation process comprising the step of cooling the granules in a cooling fluid bed (F 2 ). At least part of the fluidizing air coming out from said cooling fluid bed (F 2 ) is fed into the granulation fluid bed (F 1 ).

FIELD OF APPLICATION

In its most general aspect, the present invention refers to a fluid bedgranulation process of an appropriate substance like, for example, urea,ammonium nitrate, ammonium chloride and similar substances susceptibleto being granulated.

In particular, this invention concerns a fluid bed granulation processcomprising control of the temperature of said bed through injectiontherein of a flow of hot air.

The invention also refers to a granulation apparatus, used to carry outthe aforementioned process.

PRIOR ART

It is known that in a fluid bed granulation process, the obtainment of apredetermined substance takes place through continuous growth (in volumeand in mass), of granule seeds of such a substance, continuously fedinto said fluid bed, at the same time with a flow of an appropriategrowth substance in liquid state.

Generally, the growth substance is of the same nature as the substanceto be granulated and is in liquid form, suitable for wetting, adheringand solidifying on the seeds and on the growing granules which,together, constitute said fluid bed.

Said growth substance is fed to the fluid bed at a high temperature sothat the growth substance itself, once solidified on the seeds, can keepadherence characteristics such as to allow adhesion to the granule offurther growth substance while it is inside the fluid bed.

Moreover, inside the fluid bed it is necessary to maintain thetemperature within predetermined and, generally, relatively high valuesin order to allow the evaporation of the solvent present in the growthsubstance which is generally fed in solution, for example aqueous in thecase of urea, into said fluid bed.

The temperature of the fluid bed must be chosen also taking into accountthe fact that a possible cooling of the growth substance, before itscontact with the seeds and with the growing granules, could determine apremature solidification thereof, with consequent difficulties, if notactually impossibility, in adhesion to the granules and with formationof powders which require subsequent recovering.

To satisfy the aforementioned requirement, in other words to control andadjust the temperature of the fluid bed within predetermined values, ithas been proposed to feed, into the granulation fluid bed itself, anadditional flow of appropriately hot air, which is preferably injectedat the same level as the flow of growth substance.

Moreover, during the start-up step, or in operation with low loads, orelse when the fluidification air, which is used with the very high flowrates required for the formation and maintenance of the fluid bed, is ata particularly cold room temperature, it becomes necessary to carry outa suitable preheating of such fluidification air through suitable heatexchangers outside the fluid bed.

Although advantageous from some points of view, such a suggestionsuffers from a serious recognized drawback.

Indeed, due to the very high air flow rates circulating in the fluidbed, a control of the temperature according to the aforementionedsuggestion necessarily involves very high energy consumption for thepreheating of the fluidification air (when required) and of theadditional flow of air. This energy consumption has a negative impactupon the operating costs of the process.

The presence of such apparatuses for preheating the air also have anegative impact upon the embodying costs and the structural complexityof the corresponding granulation plant.

SUMMARY OF THE INVENTION

The technical problem underlying the present invention is to devise andto make available a fluid bed granulation process of the type consideredabove, having functional characteristics so as to overcome all the citeddrawbacks linked to the prior art and, in particular, such that thetotal energy consumption necessary to keep the fluid bed at apredetermined temperature which ensures the optimal completion of theprocess is substantially reduced.

The problem is solved according to the invention by a fluid bedgranulation process of an appropriate substance, with control of thetemperature of said bed, comprising a cooling step of the finished hotgranules thus obtained, in a respective cooling fluid bed, characterizedin that at least a part of the fluidification air coming out from saidcooling fluid bed of the finished granules is fed into the granulationfluid bed.

Preferably, all of the fluidification air fed into the granulation bedcomes from the cooling bed.

Advantageously, substantially all of the fluidification air coming outfrom the cooling bed is used as fluidification air of said granulationbed.

Even more advantageously, the fluid bed granulation process of thepresent invention is characterized in that it uses one single flow offluidification air to continuously form and support, in order, saidcooling and granulation fluid beds, substantially arranged in serieswith respect to said single flow and in fluid communication with eachother.

Further characteristics and advantages of the invention will becomeclearer from the detailed description of an embodiment of a fluid bedgranulation process according to the invention, given hereafter withreference to the attached drawings, for indicative and non-limitingpurposes.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 schematically shows an axonometric view of an apparatus forcarrying out the fluid bed granulation method of the present invention;

FIG. 2 schematically shows a cross section of the same apparatus of FIG.1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to the figures, an apparatus for carrying out a fluid bedgranulation process according to the present invention is globallyindicated with 1.

Such an apparatus comprises a self-supporting structure 2, substantiallyin the shape of a parallelepiped container, which defines a space Ainside it, in which two fluid beds F1 and F2 are intended to berealized, as can be seen more clearly in the rest of the description.

Said container structure 2 (which hereafter shall simply be called:container 2), has long side walls 5, 6, short front 7 (or top) and rear8 walls; it is closed at the top by a conventional and thereforeunrepresented cover, and it is equipped at the bottom with a double baseplate, 4, 4 a, upper and lower respectively.

In accordance with a characteristic of the present invention, the topwall 7, of said container 2, has the bottom side 7 a, spaced from thebase plate 4, of said double base plate, with which it thus defines apassage (or port) 20, which places the space A in communication with theoutside of said container 2. Moreover, in accordance with anothercharacteristic of the present invention, the aforementioned base plates4, 4 a, extend from the rear wall 8, of the container 2, up to past saidtop wall 7, for a predetermined length portion. At their free frontends, to a front panel 17 is fixed to the base plates 4, 4 a, preferablysubstantially parallel to the top wall 7, with which it constitutes asort of pocket 18, in the example of the figures extending for the wholewidth of said wall 7 and in communication with the space A, through theaforementioned passage 20.

The base plates 4, 4 a of said double base plate, the rear wall 8 of thecontainer 2 and the front panel 17 define a chamber 19 that is in fluidcommunication with the space A right through said base plate 4, providedperforated, grated or in any case permeable to gas flows. Said chamber19, extending below the space A, is of limited height and is intended toconstitute a chamber for uniform distribution of a flow offluidification air coming into said space A, as will better turn outfrom the following of the description.

Advantageously and in accordance with a further characteristic of thepresent invention, said distribution chamber 19 has a tapered profilestarting from the rear wall 8 of the container 2, towards the frontpanel 17. For such a purpose, the base plate 4 a is provided tilted onthe opposite base plate 4, and converging on it towards theaforementioned front panel 17.

Inside said container 2 a rectangular vertical panel 15 is supported,parallel and in a predetermined spaced relationship from the rear wall 8of said container 2, with which it defines an interspace 16.

Said panel 15 is fixed to the opposite long walls 5 and 6 and to the topwall 3 of said container 2, whereas it has the horizontal bottom side 15a spaced from the base plate 4, so as to define with it a passage (orport) 15 a, suitable for placing said interspace 16 in communicationwith the space A inside the container itself. The interspace 16 is incommunication with the space A, also close to the upper wall of saidcontainer 2, through an opening 11.

Inside the container 2 and at a predetermined distance from its baseplate 4, a rectangular shelf 14 is positioned, perimetrically fixed tothe long sides 5, 6, to the front wall 7, of said container 2 and to theaforementioned panel 15. Said shelf 14, defines in said space A, agranulation zone B inside the space A and is intended to support thegranulation bed F1 of a predetermined substance; for such a purpose theshelf 14 is perforated, grated or in any case made permeable to a flowof fluidification air, necessary for the formation and maintenance ofsaid bed F1.

In FIG. 1, a distributor device (per se known) of seeds of granules ofthe substance to be granulated, positioned in the container 2, at itstop, is schematized with 10, whereas distributor-supplier devices ofgranule growth liquid substance (also known and therefore notrepresented in detail) are schematized with 12 and 13.

In FIG. 2, an opening is schematized with 22, associated with the rearwall 8, for the entry of air inside the chamber 19. Such an opening 22is in fluid communication with per se known, and therefore notrepresented, means to blow the air into said chamber 19.

With reference to the apparatus of FIGS. 1 and 2, an embodiment of thegranulation process of the present invention shall now be described.

By feeding a continuous flow of seeds of granules of a predeterminedsubstance and at the same time a continuous flow of growth substanceinto the granulation zone B, a granulation fluid bed F1 is formed on theshelf 14. This granulation bed is obtained, supported and maintainedthrough a continuous flow of fluidification air, fed into the chamber 19and from here, through the base plate 4, into the space A, below saidshelf 14. Corresponding to the proceeding of the granulation (growth ofgranules) there is an increase in the height of the fluid bed F1, untilits free surface reaches the (precalculated) level of the opening 11. Atthis point, through said opening 11, which substantially acts as a weir,begins a continuous pouring (or “discharge”) from the bed F1 to theinterspace 16, of granules of substance which are very hot (theirtemperature depends upon the temperature of the growth substance) andabove all which are finished, in other words of predetermined grainsize.

From the start of such pouring onwards the height of the granulation bedF1 remains substantially constant.

The finished granules, continuously passed in the interspace 16, “fall”in a substantially guided manner, or in a cascade, on a fluid bed F2comprising finished granules 15 arranged for the starting step adjacentto the perforated base plate 4, where they are subjected to theaforementioned flow of fluidification air used for the bed F1. On such abase plate 4 a second fluid bed F2 is thus defined, consistingexclusively of finished granules, which extends in said space A, on saidbase plate 4, in the interspace 16 and in the pocket 18, whichcommunicate with said space.

On the free surface of the fluid bed F2 at the interspace 16 and thepocket 18 there is a lower pressure with respect to what can be measuredon the free surface of the fluid bed F2 at the chamber A, between thewalls 7 and 15; for this reason, and since the three quoted zones arefunctionally comparable to communicating vessels, the height of thefluid bed F2 in the interspace 16 and in the pocket 18 is greater withrespect to that between the walls 7 and 15, on said base plate 4.

It should be noted that the cooling fluid bed F2 is in fluidcommunication with the overlying granulation bed F1 exclusively throughthe shelf 14, for supporting said bed.

It should also be noted that the aforementioned interspace 16 carriesout a duct function, so-called downcomer, for the transfer of thegranules from bed F1 to bed F2.

In the fluid bed F2, the hot finished granules exchange heat with theflow of fluidification air, fed at room temperature. While the finishedgranules cool down, this air is heated by them. And this same heatedair, coming out from the fluid bed F2, is used as fluidification air ofthe granulation bed F1, to which it is fed.

For such a reason, the zone of space A between said base plate 4 and theoverlying shelf 14, is known as the granule cooling zone and, at thesame time, it can be considered the preheating zone of thefluidification air of the granulation bed F1.

Feeding preheated fluidification air to the granulation bed means, onthe one hand, providing the amount of air necessary for the formationand maintenance of said fluid bed and, on the other hand, giving to thissame bed the amount of heat necessary to decrease or even preventpremature solidification of the growth substance and, at the same time,to allow the evaporation of the solvent possibly present in the growthsubstance fed, in solution, into the growth fluid bed.

Using the air coming out from the granule-cooling fluid bed as preheatedfluidification air for the granulation bed also means reducing the totalair consumption to complete the granulation process.

The height of the fluid bed F2 (granule cooling bed and fluidificationair preheating bed) is such that its free surface in the pocket 18reaches the upper edge of the front panel 17, ensuring the discharge, tothe outside of the container 2, of finished and cooled granules.

Since the fluid bed, as is well known, has behavior totally comparableto that of a liquid, the levels of the granules in the pocket 18, in theinterspace 16 and in the space A are stabilized at the respectivepiezometric heights.

It should therefore be noted that the height of said front panel 17, bydetermining the height of the fluid bed F2, also determines the averagepermanency time of the finished and hot granules in the cooling zoneand, consequently, determines both the temperature of the finishedgranules discharged from the container 2 and, above all, the preheatingtemperature of the fluidification air.

From the start of the aforementioned “discharge” of finished granules,the process of the present invention and the relative apparatus are inoperation.

At this point a basic characteristic of the process of the presentinvention should be noted: the beds F1 and F2, respectively forgranulation and cooling of the finished granules/preheating offluidification air, are formed and supported by a same flow offluidification air, with respect to which said beds F1 and F2 arearranged substantially in series.

A second characteristic of the aforementioned process is that thefinished and hot granules are poured substantially in cascade from saidgranulation bed to the cooling bed.

The main advantage achieved by the present invention consists, asstated, of the large saving in energy consumption with respect to whathas been required up to now to carry out fluid bed granulation processesof the prior art, wherein a temperature control is provided throughintroduction of an additional hot air flow or by pre-heating, by meansof heat exchangers, of the fluidification air itself duringpredetermined situations of the operating cycle of the granulationprocess. Taking into account the considerable amounts of fluidificationair and additional hot air involved in the aforementioned processes, theaforementioned energy saving translates into an equally substantialreduction in the operating costs of the process.

This advantage is made possible thanks to the use of a single flow ofair for the fluidification of the cooling bed of the finished granulesand, in the order, of the granulation bed, which involves an effectivepreheating of the air subsequently fed to the latter operation.

According to a preferred embodiment, the front panel 17 comprises amobile bulkhead 21, adjustable in height (able to slide vertically).

By varying the height of such a front panel 17, the height of the secondfluid bed F2 is consequently varied. This means that, if for example theheight of such a front panel 17 is increased, the height of the secondfluid bed F2 increases and, consequently, the average permanency time ofthe granules inside such a bed also increases.

This entails an improved heat exchange between the aforementionedgranules and the fluidification air, which can increase its entrytemperature into the granulation bed F1.

The invention thus, conceived is susceptible to further variants andmodifications all of which fall within the inventive concept and, assuch, fall within the scope of protection of the invention itself, asdefined by the following claims.

For example, the pocket 18 and the interspace 16 can be realized with awidth lower than the width of the corresponding short front wall 7, ofthe panel 15 respectively.

1. Fluid bed (F1) granulation process of a predetermined substance atcontrolled temperature, comprising the steps of removing the finishedhot granules from said granulation fluid bed (F1), cooling down saidgranules in a cooling fluid bed (F2), continuously formed and supportedby a respective flow of fluidification air, characterized in that atleast part of the fluidification air coming out from said cooling fluidbed (F2) of the finished granules is fed into the granulation fluid bed(F1).
 2. Granulation process according to claim 1, characterized in thatall of the fluidification air fed into the granulation bed (F1) comesfrom the cooling bed (F2).
 3. Granulation process according to claim 1,characterized in that substantially all of the fluidification air comingout from the cooling bed (F2) is used as fluidification air for saidgranulation bed (F1).
 4. Fluid bed (F1) granulation process of apredetermined substance at controlled temperature, comprising a step ofcooling finished hot granules in a respective cooling fluid bed (F2),characterized in that it uses one single flow of fluidification air tocontinuously form and support, in order, said cooling and granulationfluid beds (F1, F2), substantially arranged in series with respect tosaid single flow.
 5. Granulation process according to claim 4,characterized in that the finished granules of said substance aretransferred substantially in a cascade to said cooling fluid bed (F1).6. Apparatus for carrying out the fluid bed granulation process atcontrolled temperature of claim 4, comprising a self-supportingstructure (2) substantially shaped like a container, defining agranulation space (A) inside of it, in which a shelf (14) is positioned,intended to support a granulation fluid bed (F1), characterized in thatit comprises, in said space (A), a further base plate (4), positionedbelow and in a predetermined distanced relationship from said shelf(14), said base plate (4) being intended to support a respective coolingfluid bed (F2) of hot finished granules coming from said granulation bed(F1), said cooling bed (F2) being in fluid communication with saidgranulation bed (F1) through said shelf (14), provided perforated,grated or in any case permeable to gas flows, a downcomer (16),extending vertically in said space (A), suitable for the transfer offinished granules from said granulation fluid bed (F1) to said coolingfluid bed (F2) at said further base plate (4), means for feeding anddistributing (22, 19) fluidification air in said space (A) below saidfurther base plate (4), to form and maintain said cooling bed (F2) andsaid granulation bed (F1), which are arranged in series with respect tosaid flow.
 7. Apparatus according to claim 6, characterized in that saiddowncomer (16) comprises a vertical panel (15), supported in said space(A) in a predetermined spaced relationship from a wall (8) of saidcontainer structure (2), defining with it an interspace (16), said panel(15) having a horizontal bottom side spaced from said further base plate(4), so as to define with it a passage (15 a), suitable for putting saidinterspace (16) in communication with the space (A) above theaforementioned base plate (4).
 8. Apparatus according to claim 7,characterized in that said interspace (16) is in communication at thetop with said space (A), through an opening (11) provided in it. 9.Apparatus according to claim 6, characterized in that said cooling fluidbed (F2) is in communication with the outside through a pocket (18)comprised between a wall (7) of said container structure (2) and a frontpanel (17) fixed to the base plate (4) supporting the cooling bed (F2)and preferably parallel to said top wall (7).
 10. Apparatus according toclaim 9, characterized in that said front panel 17 comprises a mobilebulkhead (21), adjustable in height.